This invention relates to optical sensors for determining the position of one or more moveable surfaces where the surfaces comprise patterned regions of high and low reflectivity to electromagnetic radiation (EMR). Such moveable surfaces are found in displacement sensors, angle sensors and torque sensors.
Typically, such sensors consist of at least one EMR source illuminating a patterned region on at least one moveable surface, at least one lens or other focussing means to focus the EMR reflected from the surface(s), and at least one EMR sensitive array to receive the focussed image. The pattern consists of regions of high and low reflectivity to the EMR emitted from the source(s), and is typically either marked on, attached to, or otherwise applied to the respective moveable surface. The pattern may have a constant period, but may also encrypt absolute position information via a formatted barcode. Such sensors also require means of processing the image to decrypt meaningful information relating to the position of the moveable surface(s), and also means of outputting this information. In co-pending International Patent Application No. PCT/AU98/00645 these functions are achieved by the array architecture forming part of an Application Specific Integrated Circuit (ASIC). In the present specification, the combination of all these components, including the respective moveable surface(s), will be termed a xe2x80x9csensing systemxe2x80x9d.
In the case of linear displacement sensing systems, the pattern is applied to the surface of a translating plate. An example of a linear displacement sensing system using this principle is disclosed in International Patent Publication WO97/03338. In this patent the moveable surface has two patterns, each of high and low reflectivity. A coarse pattern is used for gross position measurement and a fine pattern for accurate position measurement.
In the case of angular displacement sensing systems, the pattern is usually applied to a cylindrical surface, with the illumination and reflection of EMR occurring in a substantially radial direction, or on a disk-like surface, with the illumination and reflection of EMR occurring in a substantially axial direction. Many other axisymmetric shapes can also be used. An example of an angular displacement sensing system of the type described is disclosed in German Patent Application DE19705312. In this patent the arrangement of EMR illumination, focussing lens, array and processing architecture is clearly shown.
A torque sensing system can also be constructed by utilising multiple arrays (or, for example, a single two dimensional array) and having patterns which change position circumferentially relative to each other as a function of applied torque. An example of such a device is disclosed in co-pending International Patent Application No. PCT/AU98/00645. This patent shows a number of possible arrangements of utilising the basic principle described above to measure torque.
In all of these embodiments, the sensing system is supported within an enclosure. This enclosure serves to eliminate contamination of the moveable patterned surface(s) by foreign material or extraneous EMR. The electrical components which make up the sensing system, are usually mounted within this enclosure on a Printed Circuit Board (PCB), while the other fixed components such as the focussing lens (and ancillaries) are usually mounted separately within the enclosure. The fixed components of the sensing system require accurate alignment to each other and, in turn, these require to be correctly spatially positioned with respect to the moveable surface(s). All previous designs of such devices have required disassembly of the enclosure to repair or replace all or part of the sensing system which, if not carried out by skilled personnel, may result in corruption of this alignment and spatial positioning of the fixed and moveable sensing system components. This, in turn, will dramatically degrade the optical and electronic performance of the sensing system.
The essence of the present invention resides in the provision of a removable sensor module containing all the fixed components of the sensing system. These fixed components are therefore accurately mutually aligned and integrated within the housing of the sensor module. The sensor module can be then installed into the enclosure, the latter surrounding the moveable surfaces of the sensing system, via an aperture in the enclosure designed to provide accurate xe2x80x9cdatumsxe2x80x9d for mounting the sensing module. In this way the fixed components of the sensing system (in the sensor module) are aligned with, and accurately spatially mounted with respect to, the moveable surface(s) within the enclosure. Moreover, replacement of the fixed components of the sensing system can now be readily achieved by untrained personnel via replacement of the sensor module as a single component, while maintaining accurate alignment and spatial positioning of the fixed and moveable components of the sensing system.
The present invention consists of a sensor for determining the position of a moveable surface having patterned regions of high and low reflectivity to EMR, the sensor comprising an ASIC, at least one lens, and at least one EMR source, the ASIC comprising at least one array of EMR sensitive detectors and processing means, the EMR source facilitating illumination of the surface and the at least one lens facilitating the focussing of reflected EMR from the surface and generating an image on the at least one array of EMR sensitive detectors corresponding to the pattern on the surface, characterised in that the ASIC, the at least one lens, and the at least one EMR source are all enclosed in a single housing providing accurate optical alignment of these elements and integrated as a single replaceable module, and the processing means of the ASIC facilitates processing of the image to determine the position of the pattern on the surface.
It is preferred that the housing of the sensor also comprises an electrical connector and the processing means also facilitates the outputting of a digital or analog electrical representation of the position to the electrical connector.
It is preferred that the electrical connector comprises a multi-pin plug.
It is preferred that the EMR emitted by the at least one EMR source passes through a light guide.
It is preferred that the multi-pin plug also provides electrical power to the sensor.
It is preferred that the at least one lens forms part of a lens system, the lens system comprising at least two lenses separated by an iris.
It is preferred that the at least one lens is any one of a refractive, reflective or diffractive optical component.
It is preferred that the at least one EMR source comprises a Light Emitting Diode (LED).
It is preferred that the ASIC is mounted on a PCB and the PCB is mounted in the housing.
In some embodiments of the present invention all the opto-electronic components which make up the sensor module, including the ASIC and LEDs, are mounted on a single PCB using Surface Mount Devices (SMDs). The LEDs may have focussing lenses integrated into their bodies or, alternatively, one or more light guides may be used to convey EMR from the LEDs to the moveable surface(s) in order to minimise optical losses. This light guide may consist of a moulded transparent plastic tubular or solid section or, alternatively, fibre optic technology may be employed.
Opto-electronic position sensing systems, of the type described in reference to the present invention, rely on a discrete (ie. non-continuous) image sampling process. It is therefore preferred that the LEDs are intermittently pulsed according to a predetermined duty cycle with a high xe2x80x9conxe2x80x9d current for a very short time period. The xe2x80x9conxe2x80x9d current can in fact equate to many times the steady state current capability of the LEDs. This allows much higher instantaneous optical power emissions to be achieved without damaging the LEDs. It also xe2x80x9cstrobesxe2x80x9d the pattern on the moveable surface(s) and hence reduces xe2x80x9csmearingxe2x80x9d of the focussed image on the array(s) at higher velocities of these surface(s). It is preferred that the control of the duty cycle of the LEDs is also included in the ASIC architecture.
Preferably the at least one array, which forms part of the ASIC, is either a one or two dimensional array and uses either photodiode or Charged Couple Device (CCD) technology.
The at least one lens focuses the reflected EMR from the moveable surface and produces a sharp image on the array(s) in the ASIC. The mounting of the lens(es) inside the housing of the sensor module ensures that its focal properties and the geometric relationship between the moveable surface(s), the lens(es) and the ASIC are not disturbed if the sensor module is removed or replaced. The lens may comprise a classical curved refractive component which focusses EMR transmitted through the lens material. Alternatively the lens may comprise a reflective curved component which focusses impinging EMR reflected from the internal or external surface of the lens material. Either of these refractive or reflective lenses may have optical surfaces consisting of simple continuous curved surfaces (e.g. spherical or parboloidal) or, alternatively, the surfaces may be discontiuous in the form of a Freznel arrangement. In a further alternative embodiment the lens may be arranged as a diffractive component.
Where the sensor module forms part of the earlier referred to linear displacement sensing systems, linear displacement, velocity and acceleration of the moveable surface can also be calculated by analog and/or digital processing on the ASIC based on the position of the moveable surface at each time sample.
Where the sensor module forms part of the earlier referred to angular displacement sensing systems, angular displacement, angular velocity and angular acceleration can also be calculated by analog and/or digital processing on the ASIC based on the position of the cylindrical, disk-like or otherwise axi-symmetric moveable surface at each time sample.
Where the sensor module forms part of the earlier referred to torque sensing systems, torque, rate of change of torque and torque acceleration, in addition to those variables mentioned above in reference to angular displacement sensing systems, can also be calculated by analog and/or digital processing on the ASIC. In this embodiment the ASIC typically has at least two one dimensional arrays, at least one array detecting each pattern. Alternatively a single two dimensional array may be incorporated on the ASIC. The torque-based variables mentioned above are calculated at each instant of time by measuring the differential position of two separated moveable surfaces connected by a member of predetermined torsional stiffness. Suitable processing algorithms are disclosed in co-pending International Patent Application No. PCT/AU98/000645 and enable measurement of torque in a stationary as well as a rotating shaft and also, for certain embodiments employing bar coded patterns, the absolute angular position of a stationary as well as a rotating shaft.
It is preferred that the processing means provides a fixed protocol for output from the electrical connector of the sensor module. There are many known protocols for communicating the output to other devices, for example analog voltage, pulsed width modulation (PWM) and various serial protocols can be used. The output protocols can also be xe2x80x9csplitxe2x80x9d between a number of electrical output conductors within the electrical connector, that is between the wires or plug pin connections. In one embodiment one output protocol of the sensor module is an analog voltage proportional to torque on one conductor, and a simultaneous serial output protocol communicating angular displacement on another conductor. In another embodiment a single high-level serial output protocol is used to output the value of all variables from the sensor module, such as a Controller Area Network (CAN) protocol.
It is preferred that the EMR source(s) and ASIC be powered from the same voltage source. This source preferably connects to the sensor module via the same electrical connector which contains the conductors for output communication. This electrical connector may be integrated as part of the sensor module housing or, alternatively, may be at the end of a wiring harness and remote from the sensor housing if required.
It is preferred that the sensor module housing is injection moulded from an engineering plastic and the internal components of the sensor module moulded xe2x80x9cin situxe2x80x9d. This provides rigid support to these internal components, as well as preventing unauthorised disassembly of the sensor module, the latter which may disturb the alignment and spatial positioning of these internal components.